--- a/content/media/MediaResource.h
+++ b/content/media/MediaResource.h
@@ -417,17 +417,17 @@ public:
                   MallocSizeOf aMallocSizeOf) const MOZ_OVERRIDE
   {
     // Might be useful to track in the future:
     // - mChannel
     // - mURI (possibly owned, looks like just a ref from mChannel)
     // Not owned:
     // - mDecoder
     size_t size = MediaResource::SizeOfExcludingThis(aMallocSizeOf);
-    size += mContentType.SizeOfIncludingThisIfUnshared(aMallocSizeOf);
+    size += mContentType.SizeOfExcludingThisIfUnshared(aMallocSizeOf);
 
     return size;
   }
 
   virtual size_t SizeOfIncludingThis(
                   MallocSizeOf aMallocSizeOf) const MOZ_OVERRIDE
   {
     return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);